The present invention relates to laser-based information recording systems and, more particularly, to such systems providing for rewriting on magneto-optical media. A major objective of the present invention is to provide an improved laser-based information recording system which can write without requiring separate read or erase steps.
Magneto-optical systems combine the high information storage densities achievable using optical media with the rewrite capability characteristic of magnetic recording systems. A magneto-optical medium typically includes a storage layer with a large number of magnetic domains, each of which can store a bit of binary data in the form of an upward or downward spin orientation. The polarization of laser light reflected by such a domain is rotated, either clockwise or counter clockwise, as a function of the spin orientation of the domain due to a "magneto-optic effect".
A magneto-optical medium can be read by illuminating each domain with linearly polarized light and analyzing the reflection from each domain using a polarization analyzer and an intensity detector. The detector and axis of the polarization analyzer are aligned so that maximal reflected light is detected when, for example, the magneto-optic effect has resulted in a clockwise rotation in the reflected linearly polarized light. Linearly polarized light rotated in the opposite direction results in a lower intensity at the detector. Thus intensity at the detector represents the direction of rotation of linearly polarized light, which in turn represents the spin orientation of the domain reflecting the light. Thus data encoded in the form of spin orientations can be read out in the form of light intensities. More sophisticated systems use differential sensing of complementary polarization states to improve common mode rejection.
Domain spin orientation can be thermomagnetically controlled. The preferred magneto-optic materials are amorphous ferrimagnetic rare-earth transition metal thin films enclosed by glass or plastic protective layers. The ferrimagnetic film materials are selected with perpendicular uniaxial anisotropy so that magnetization lies perpendicular to the film plane. Preferably, the compensation point, where coercivity is maximal, is near the predetermined ambient temperature to ensure that external fields and the read process itself do not disturb stored information. For these materials, coercivity (Hc) is high, e.g., several thousand Oersteds (kOe), at ambient temperature, but relatively low e.g., a few hundred Oersteds, within an elevated temperature range. The spin orientation of a domain can be established by a local magnetic field while the domain is heated. Heat can be applied locally using a laser pulse to select the spin orientation of a single domain without affecting neighboring domains.
The challenge of magneto-optical recording is to achieve direct, bit-by-bit, overwrite capability. In other words, a section of medium should be rewrittable without prior erasure and without respect to what information was previously contained in that section. Simplicity and speed of the writing process are critical in view of the great storage densities, on the order of 10.sup.8 bit/cm.sup.2 or more, and the resulting vast storage capacities, measured in gigabytes, attainable using magneto-optical media.
Difficulties in achieving high-frequency switching of a magnetic field over large areas of a medium constrain writing speed. Systems employing high speed magnetic switching are discussed in the background to U.S. Pat. No. 4,649,519 to Sun et al. This switching limitation can be avoided by writing in two passes, one for each magnetic field direction. In each pass, a write laser is modulated to illuminate only those domains to be oriented along the magnetic field present. Typically, the first pass is an erase, in which the section of the medium to be rewritten is initialized to one orientation using a constant magnetic field and light intensity. The magnetic field is then reversed and the light modulated to switch the orientations of selected domains.
The requirement for two passes renders the rewriting process unduly slow. At a considerable cost in complexity, the speed problem can be ameliorated by a systems approach as described in "Magneto-Optic Recording Erase Method", IBM technical disclosure Vol. 29, No. 5, Oct. 1986. This systems approach provides for certain operations to be delayed until "free time" is available. However, in applications such as "video interactive" and disk-based memory caching, such free time may not be available.
More complex magneto-optical media have been used to eliminate the requirement for two-pass rewriting. In addition to including the standard magneto-optical storage layer, these media include a magnetic control layer in which magnetic domains are formed to provide localized biases during a write operation. One-pass rewriting is implemented without requiring highspeed magnetic switching.
One such system is disclosed in German Pat. No. 3,619,618, assigned to Nippon Kogaku, K.K. This system uses a media with a control (reference) magnetic layer below the read (memory) layer. Prior to a write operation, all the domains of the control layer are initialized to an initial orientation using a relatively strong external magnetic field. This external field, which does not affect the data layer, can be applied by an electromagnet outside the medium. During a write operation, the control layer is exposed to a second external magnetic field with an orientation opposite to the initializing field. This second external field is relatively weak so that it does not affect the orientation of a control domain unless that domain is heated to its Curie temperature. One magnetic orientation is imposed on a storage domain by heating it above its Curie temperature so that the associated control domain remains below its Curie temperature by using a relatively low power write beam. The opposite magnetic orientation is imposed on a storage domain by heating both it and the associated control domain above their respective Curie temperatures. The storage domain assumes the orientation of the control domain which is determined by the magnetic orientation of the externally applied field. Those control domains inverted during the write operation are reinitialized to the initial magnetic orientation before the next write operation. While this system does allow direct bit-by-bit overwrite of the storage layer, two external magnets are required to set and then clear domains of the control layer. The storage and control layers are in contact so that the incorporating system relies on exchange interaction, rendering the system difficult to manufacture and sensitive to ambient temperature variations.
Systems requiring neither external magnetic elements nor separate erase steps for either the read or control layers have been disclosed in U.S. Pat. No. 4,649,519 to Sun et al. and by Han-Ping Shieh and Mark H. Kryder in "Magneto-Optic Recording Materials with Direct Overwrite Capability", Applied Physics Letters, Vol. 49, No. 8, Aug. 25, 1986. Both these systems employ write techniques which operate to invert the spin orientation domain from its previous state. The advantage of these systems is that the spin orientation can be controlled by flipping or not flipping as required. The disadvantage of this approach is that the prior state of the domain must be known prior to writing. This can be achieved in a single read-before-write pass, but it still requires the separate read procedure. In addition, a source of error is introduced since misreading a domain forces a write error. Furthermore, since writing has to occur exactly at the same place every time, timing constraints are impracticably severe. Thus, these write-after-read systems are disadvantageous in that they are more complex and more prone to error than is desirable.
What is needed, which has not been provided heretofore, is a system which provides the durability and storage capabilities of optical media and the recording convenience of magnetic media. It should not require separate erase operations between write operations, either in a storage layer or in a control layer. It should not require reading prior state before writing. In addition, it should not require high speed switching of an external magnet. In short, what is needed is an optically readable media which does not require separate erase and read steps during recording.